Preheat the oven to 375°F.
In a bowl, toss the crostini with the olive oil and chopped olives. Arrange on a baking sheet and toast for 8-10 minutes, until lightly brown.
Increase the oven to 400°F.
Wrap the garlic in tin foil, drizzle with olive oil, and roast until very tender and fragrant, about 20 minutes. Remove the roasted garlic cloves from the head and place in a blender, along with all other dressing ingredients except the oil. Purée until smooth, adding water if necessary. With the motor running, drizzle in the oil until emulsified. Adjust the seasoning
Toss the lettuce in the lemon dressing, top with manchego and serve with olive crostini.